1. Field of the Invention
This invention relates to an exposing apparatus used for the manufacture of semiconductor elements, and in particular to a reticle alignment optical system in a projection type exposing apparatus.
2. Description of the Prior Art
Heretofore, generally in a reduction projection type exposing apparatus, the images of a plurality of alignment marks provided on a reticle have been formed on a slit in an alignment optical system, and alignment of the reticle has been accomplished by the photoelectric signals from photoelectric detectors disposed rearwardly of the slit, with the slit as the reference. Such alignment is effected on the premise that the slit in the alignment optical system is kept in a strictly predetermined positional relation to the movement of a stage for supporting thereon a wafer as an exposed object on which the pattern on the reticle is printed by means of a projection objective. For this reason, it is necessary that by the use of a reference mark (fiducial mark) provided on the stage on which the wafer is placed, one slit in the alignment optical system and the fiducial mark be optically brought into coincidence with each other through the projection objective, whereafter the stage is moved, whereby the alignment optical system is adjusted so that the fiducial mark and another slit in the alignment optical system are coincident with each other through the projection objective.
In this case, the interval between the plurality of alignment marks on the reticle is determined to a predetermined value and therefore, in order to bring the fiducial mark into coincidence with the slit corresponding to one alignment mark and thereafter bring the fiducial mark into coincidence with the slit corresponding to the other alignment mark, the stage is moved by a value corresponding to the product of the value of the interval between the alignment marks and the projection magnification. Where the other alignment mark and the fiducial mark are not coincident with each other, the alignment optical system is adjusted by Harbing glass (a parallel flat plate rotatable to vary the inclination thereof relative to the optical axis) so that the two are coincident with each other. It is very important in alignment of the reticle to accurately align the slits in the alignment optical system relative to the running of the stage in this manner.
However, when the optical position adjustment of each slit in the alignment optical system is to be effected by the fiducial mark through the projection objective, if the magnification of the projection objective is varied from a standard magnification by some cause or other, it is difficult to accomplish accurate alignment even if the stage is moved by the value of the product of the predetermined value of the interval between the alignment marks and the projection magnification, and this has led to the possibility of an error. Where, as shown in the plan view of FIG. 8 of the accompanying drawings (which shows the positional relation on the surface of the reticle R), the alignment marks on the reticle comprise a (+) mark and a (-) mark each centered along an edge of the reticle, namely, at a position along a line orthogonal to the optical axis Ax of the projection objective, if the projection magnification of the objective is varied from .beta. to .beta.' by some cause or other, the position of the fiducial mark on the reticle moves from F.sub.Y indicated by solid line to F.sub.Y ' indicated by dotted line. Since the distance between the alignment marks formed on the reticle is determined to a predetermined value, the stage is moved by amounts x and y corresponding to the multiples of this value, but if the magnification of the projection objective is varied to .beta.', the position of the fiducial mark on the reticle is displaced not only in X direction but also in Y direction. If the reticle is to be aligned by the alignment optical system adjusted in such a state, the position of the reticle cannot be accurately aligned relative to the apparatus body and particularly, a rotational error of the reticle will occur. Thus, an error will occur during the superposition printing of a plurality of patterns on the wafer and it will become impossible to manufacture semiconductor elements having minute patterns high in degree of integration.